JP3227649B2 - Surface acoustic wave filter - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a surface acoustic wave filter comprising a plurality of SAW resonators on a piezoelectric substrate, and more particularly, to a SAW filter comprising a plurality of SAW resonators forming a ladder type filter circuit. The present invention relates to a surface acoustic wave filter arranged at

[0002]

2. Description of the Related Art For example, in a mobile communication device, a surface acoustic wave filter having a plurality of SAW resonators formed on a piezoelectric substrate is known as a high-frequency band filter. For example, Japanese Patent Publication No. 56-19765 discloses a surface acoustic wave filter in which a plurality of SAW resonators are formed on a piezoelectric substrate to form a ladder-type filter circuit.

FIG. 11 is a schematic plan view for explaining a surface acoustic wave filter disclosed in the above prior art. The surface acoustic wave filter 51 is configured using a rectangular piezoelectric substrate 52. On the piezoelectric substrate 52, a plurality of S
AW resonators are formed as series arm resonators 53 and 54 and parallel arm resonators 55 and 56. That is, in the series arm formed between the input terminal 57 and the output terminal 58, the series arm resonators 53 and 54 are connected in series. A parallel arm resonator 5 is provided between the series arm and the reference potential.
5, 56 are connected. The series arm resonators 53 and 54 and the parallel arm resonators 55 and 56 are alternately arranged between input and output.

In the surface acoustic wave filter 51, each of the series arm resonators 53 and 54 and the parallel arm resonators 55 and 56 has
Inter digital transducer (hereinafter, ID)
T) IDT5 having 53a, 54a, 55a, 56a
Reflectors 53b, 53c to 56b, on both sides of 3a to 56a,
56c is formed.

That is, in the surface acoustic wave filter 51,
Each of the series arm resonators 53 and 54 and the parallel arm resonators 55 and 56 is constituted by a one-port SAW resonator having reflectors arranged on both sides of the IDT.

The operation principle of the surface acoustic wave filter 51 is as follows. As shown in FIG. 12 schematically showing only the electrode portion in a plan view, the one-port SAW resonator 60 has a structure in which reflectors 62 and 63 are arranged on both sides of an IDT 61 arranged in the center. The IDT 61 has a structure in which a comb electrode 61a having at least one electrode finger and a comb electrode 61b having at least one electrode finger are arranged such that the electrode fingers are interposed.

In the one-port SAW resonator 60, the IDT
The surface wave excited at 61 is reflected by the reflectors 62 and 63 to be a standing wave, confined between the reflectors 62 and 63, and the SAW resonator 60 operates as a resonator having a high Q value. As is well known, in the impedance characteristic of the SAW resonator 60, there is a pole whose impedance decreases near the resonance frequency fr, and a pole whose impedance increases at the anti-resonance frequency fa appears.

In the surface acoustic wave filter 51, a pass band is obtained by utilizing the impedance characteristics of the one-port SAW resonator. That is, by matching the resonance frequency fr of the series arm resonators 53 and 54 with the anti-resonance frequency fa of the parallel arm resonators 55 and 56, the input / output impedance is matched with the characteristic impedance in the vicinity of the matched frequency. And thus constitutes a passband.

Since the one-port SAW resonator has the above-described impedance characteristics, the series arm resonators 53 and 54
Is very high near the anti-resonance frequency of the surface acoustic wave filter and very low near the resonance frequency of the parallel arm resonators 55 and 56.
At 1, an attenuation region having these frequencies as poles is formed.

In the surface acoustic wave filter 51, since the series arm resonators 53 and 54 and the parallel arm resonators 55 and 56 are configured as described above, the insertion loss can be reduced and the vicinity of the pass band can be reduced. Is considered to have a relatively large attenuation.

Japanese Patent Application Laid-Open No. 5-183380 discloses a configuration similar to that of the surface acoustic wave filter 51.
Further, there is disclosed a surface acoustic wave filter in which inductance is added to a parallel arm resonator to thereby broaden the band.

[0012]

In recent years, in mobile communication devices such as mobile phones, there has been a strong demand for expanded transmission frequency bands and reception frequency bands. Therefore, the band filters used in these devices are also required to have a wider band. In addition, in bandpass filters used in these devices, it is strongly required to increase the amount of attenuation in a stop band. That is, a large amount of attenuation must be obtained in the reception frequency band in the transmission filter and in the transmission frequency band in the reception filter.

By the way, in the band filter of the mobile communication device, the frequency of the above-mentioned stop band is located near the pass band. On the other hand, a surface acoustic wave filter having a ladder-type circuit operates according to the above-described principle, and therefore has a relatively large amount of attenuation in the vicinity of a pass band. It has appropriate characteristics as a bandpass filter.

However, in a surface acoustic wave filter having a ladder-type circuit, the amount of attenuation is very large near the attenuation pole. However, since the frequency range of the attenuation pole is narrow, the amount of attenuation sharply falls outside the frequency of the attenuation pole. Become smaller. That is, in the stop band near the pass band, a large amount of attenuation can be obtained near the frequency of the attenuation pole,
Since the frequency range where the attenuation is large is relatively narrow, there is a strong demand for expanding the frequency range where the attenuation is large.

In a surface acoustic wave filter having a ladder type circuit, a method of expanding a frequency region of a portion where a large amount of attenuation is provided in a stop band includes a method of increasing a capacitance ratio between a parallel arm resonator and a series arm resonator. Alternatively, a method of distributing a plurality of attenuation poles by using a plurality of parallel arm resonators and varying the resonance frequency of the parallel arm resonators has been attempted.

However, when the attenuation is increased by the capacitance ratio, there is a problem that the insertion loss is deteriorated.
Further, when the resonance frequencies of the plurality of parallel arm resonators are made different from each other as described above, impedance matching cannot be achieved within a pass band, and there is a problem that reflection loss increases.

That is, in the conventional method for expanding the frequency range where the attenuation is large in the stop band near the pass band,
There is a limit due to an increase in insertion loss, and improvement has been strongly demanded.

An object of the present invention is to provide a surface acoustic wave filter having a ladder-type filter circuit capable of expanding a frequency region having a large attenuation in a stop band on a low frequency side of a pass band without increasing insertion loss. Is to do.

[0019]

According to the first aspect of the present invention, a ladder-type filter circuit having a series arm between an input / output terminal and a plurality of parallel arms between the series arm and a reference potential is provided. as such, a plurality of series arm resonators and multiple on a piezoelectric substrate
Ri Na form a parallel arm resonator of a number of parallel arm resonator of the plurality of
Element is an input terminal and a series arm resonator close to the input terminal.
The parallel arm connected to the connection point between
Connected to the connection point between the series arm resonator and the force terminal
First parallel arms respectively arranged on both of the arranged parallel arms
Resonator and parallel arm connected to the connection point between series arm resonator
A surface acoustic wave filter element having a second parallel arm resonator disposed on the surface thereof, and a plurality of external terminals that house the surface acoustic wave filter element and are connected to the surface acoustic wave filter element. and packaging materials, input-output and the terminals and a reference potential terminal of said surface acoustic wave filter element, in the surface acoustic wave filter comprising a plurality of bonding wires in which a plurality of the external terminals are connected each of the packaging material, wherein the The electrode capacitance of the two parallel arm resonators is
Greater than twice the electrode capacitance of the first parallel arm resonator;
6 times or less, and the second parallel arm resonator
Connected to the reference potential provided on the package material
The length of the bonding wire connecting the external terminals is
A base provided on the first parallel arm resonator and a package material;
It is characterized in that it is longer than the bonding wire connecting the external terminal connected to the quasi-potential .

As described above, in a surface acoustic wave filter having a ladder-type circuit configuration, the attenuation characteristic in the stop band lower than the pass band depends on the impedance characteristic at the resonance frequency of the parallel arm resonator. Therefore, in order to expand the frequency region where the amount of attenuation in the stop band on the low frequency side is large, a plurality of parallel arm resonators are used, and these resonance frequencies are made different, so that a plurality of attenuation poles having different frequencies are dispersed and arranged. do it. However,
In this method, since the anti-resonance frequency also changes, impedance matching in the pass band cannot be achieved, and the insertion loss deteriorates.

According to the first aspect of the present invention, in view of the above problems, in a surface acoustic wave filter using a plurality of parallel arm resonators, a plurality of parallel arm resonators are connected in series using bonding wires. Add an inductance element to
The characteristic feature is that the impedance characteristics around the anti-resonance frequency of the parallel arm resonator are suppressed from changing, and the frequency range where the attenuation is large due to the dispersion of the resonance frequency of the plurality of parallel arm resonators. .

[0022] In this onset Ming, before Symbol surface acoustic wave filter element has a plurality of series arm resonators and a plurality of parallel arm resonators, an input and terminal or an output terminal of said surface acoustic wave filter element, the A first parallel arm resonator disposed in a parallel arm connected to a connection point between the series arm resonator close to the input / output terminal and a parallel arm connected to a connection point between the series arm resonators; And a second parallel arm resonator arranged, wherein the electrode capacity of the second parallel arm resonator is larger than the electrode capacity of the first parallel arm resonator.

Further, between the connected parallel arm, and the series arm resonators which are close to the output terminal and the output terminal to a connection point between the series arm resonator close to the input terminal and the input terminal The first parallel arm resonator is disposed on both of the parallel arms connected to the connection point of the first and second parallel arm resonators, and the electrode capacity of the second parallel arm resonator is equal to the electrode capacity of the first parallel arm resonator. Is greater than twice and less than or equal to six times.

Furthermore, prior SL and second parallel arm resonators, the length of the bonding wire that connects the external terminal connected to the reference potential provided on the package material, said first
Is longer than the bonding wire connecting the parallel arm resonator and the external terminal connected to the reference potential provided on the package material, thereby further expanding the frequency region where the attenuation is large. be able to.

[0025]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS A surface acoustic wave filter according to the present invention will be described below with reference to a structural example shown. FIG. 1 is a schematic plan view for explaining a structural example of a surface acoustic wave filter according to the present invention, and FIG. 2 is a diagram showing an equivalent circuit thereof.

As shown in FIG. 2, the surface acoustic wave filter 1
In this example, series arm resonators 3, 4, and 5 are inserted in a series arm formed between the input terminal 2a and the output terminal 2b.
A parallel arm is formed between the connection point 6a between the series arm resonators 3 and 4 and the reference potential, and the parallel arm resonator 7 is inserted into the parallel arm. Similarly, a parallel arm is formed between the connection point 6b between the series arm resonators 4 and 5 and the reference potential,
A parallel arm resonator 8 is connected to the parallel arm.

As will be apparent from a structural example described later, the series arm resonators 3, 4, 5 and the parallel arm resonators 7, 8 are connected by bonding wires to external terminals for connection to the outside of the package material. In this structure example, the inductances 9a to 9d by the bonding wires are inserted in series with the respective resonators.

That is, the series arm resonator 3 and the input terminal 2a
And the inductance 9a is formed by the bonding wire connecting the series arm resonator 5 and the output terminal 2b.
Are inserted in series with the series arm resonators 3 and 5, respectively. Similarly, the inductances 9c and 9d are respectively formed by the bonding wires connecting the parallel arm resonators 7 and 8 to the external terminals of the package material connected to the reference potential.
Is inserted.

In the surface acoustic wave filter 1, the electrode capacitances of the parallel arm resonators 7 and 8 are different from each other.
Is made larger than the electrode capacitance of the parallel arm resonator 7. Further, the inductance 9d of the bonding wire connecting the parallel arm resonator 8 to the reference potential is:
The bonding wire for forming the inductance 9d is relatively long so as to be larger than the inductance 9c of the bonding wire connecting the parallel arm resonator 7 to the reference potential.

The specific structure of the surface acoustic wave filter 1 shown in FIG. 2 will be described with reference to FIG. In the surface acoustic wave filter 1, a surface acoustic wave filter element 12 is arranged in a central opening of the package material 10. Although not particularly shown, a cover member (not shown) is attached so as to close the central opening 10a of the package member 10, and the surface acoustic wave filter element 12 is hermetically sealed in the package structure.

The package material 10 is made of an appropriate insulating material such as an insulating ceramic such as alumina or a synthetic resin. Steps 10b, 10b are formed on both sides of the opening 10a of the package material 10, and external terminals 11a to 11f are formed on the steps 10b, 10b.
Are formed.

The external terminals 11a to 11f are formed of a thin film or a thick film made of a conductive material such as copper or aluminum, and are not always clear in FIG. And is arranged so that it can be electrically connected to the outside by the outer surface of the package material 10. The external terminals 11a to 11f are connected to respective resonators described later of the surface acoustic wave filter element 12 by bonding wires.

The surface acoustic wave filter element 12 is configured using a rectangular piezoelectric substrate 13. The piezoelectric substrate 13 can be made of a piezoelectric single crystal such as LiNbO ₃ , LiTaO ₃ or quartz or a piezoelectric ceramic such as a lead zirconate titanate-based piezoelectric ceramic. In this structural example, the piezoelectric substrate 13 is constituted by a 36 ° Y-cut LiTaO ₃ substrate.

On the piezoelectric substrate 13, the series arm resonators 3 to 5 and the parallel arm resonators 7,
8 are formed. The series arm resonator 3 includes an IDT 3a disposed so that the electrode fingers thereof are interposed therebetween, and an IDT 3a.
1A is a one-port SAW resonator having reflectors 3b and 3c arranged on both sides of the SAW resonator. Similarly, the series arm resonators 4,
5 also has an IDT and reflectors located on both sides of the IDT. Further, the parallel arm resonators 7 and 8 also include an IDT disposed at the center and reflectors disposed on both sides of the IDT.

In the surface acoustic wave filter 1, the external terminals 11
Among the terminals a to 11f, the external terminal 11b is used as an input terminal, and the external terminal 11e is used as an output terminal. One comb electrode of the IDT 3a of the series arm resonator 3 is connected to the external terminal 11b by a bonding wire 9A. On the other hand, series arm resonators 3 and 4 are connected by conductive pattern 14a, and series arm resonators 4 and 5 are connected by conductive pattern 14b. One of the IDTs of the IDT of the series arm resonator 5 is connected to the bonding wire 9.
B connects to the external terminal 11e. Therefore, a series arm is formed between the external terminal 11b as an input terminal and the external terminal 11e as an output terminal, and the series arm resonators 3, 4, and 5 are inserted into the series arm.

On the other hand, the parallel arm resonator 7 is connected between a connection point between the series arm resonators 3 and 4 and a reference potential.
This reference potential is connected to the parallel arm resonator 7 by connecting one of the IDT electrodes of the IDT to an external terminal 11 connected to the reference potential.
The connection is made by connecting the bonding wire 9c with the bonding wire 9C. Further, the parallel arm resonator 8 is connected between a connection point between the series arm resonators 4 and 5 and a reference potential. The parallel arm resonator 8 is connected to the reference potential by connecting one of the IDTs of the IDT of the parallel arm resonator 8 to the external terminal 1 connected to the reference potential by the bonding wire 9D.
1d.

Therefore, in the surface acoustic wave filter 1, the inductances 9a to 9d shown in FIG. 2 are constituted by the bonding wires 9A to 9D. Further, as described above, the electrode capacitance of the parallel arm resonator 8 is larger than the electrode capacitance of the parallel arm resonator 7. In the present structural example, the inter-electrode opposing area in the IDT of the parallel arm resonator 8 (that is, the inter-electrode opposing area between one comb electrode and the other comb electrode) is determined by the distance between the electrodes in the parallel arm resonator 7. By setting the opposing area to 2.5 times, the electrode capacity of the parallel arm resonator 8 is set to 2.50 times the electrode capacity of the parallel arm resonator 7.
5 times.

The bonding wire 9D is longer than the bonding wire 9C. Therefore, FIG.
Is larger than the inductance 9c.

In the present invention, the electrode capacitance of the plurality of parallel arm resonators can be made different by not only changing the facing area between the electrodes as described above, but also changing the electrode finger intersection width, or changing the electrode finger crossing width. And the like can be used to change the logarithm. Alternatively, a plurality of SAW resonators having the same frequency may be connected in series or in parallel to form one parallel arm resonator, so that the electrode capacitance of the parallel arm resonator may be adjusted. That is, in the present invention, the parallel arm resonator does not necessarily need to be constituted by one SAW resonator.

Next, in the surface acoustic wave filter 1, since the electrode capacities of the parallel arm resonators 7 and 8 are different and the bonding wire 9D is longer than the bonding wire 9C, the pass band is reduced. The fact that the frequency range in which the amount of attenuation is large can be expanded in the stop band on the lower frequency side.

The equivalent circuit of the one-port SAW resonator is as shown in FIG. In FIG. 3, L1, C1, R
Numeral 1 indicates an induction component, a capacitance component, and a resistance component representing resonance when a surface acoustic wave is excited. C0 is I
Ls is the external inductance connected to the resonator, and the capacitance between the electrodes of the DT electrode finger is Ls. External inductance Ls
Does not exist, the resonance frequency fr and the anti-resonance frequency fa of the resonator are expressed by the following equations (1) and (2), ignoring the resistance component R1.

[0042]

(Equation 1)

[0043]

(Equation 2)

On the other hand, when the external inductance Ls is inserted, the resonator frequency fr 'is given by the following equation (3).

[0045]

(Equation 3)

Therefore, as is apparent from equation (3), the resonance frequency can be reduced by adding the external inductance Ls. On the other hand, regarding the anti-resonance frequency,
Even if the external inductance Ls is added, Equation (2)
Therefore, even if the external inductance Ls is inserted, the anti-resonance frequency fa does not change.

The amount of change in the resonance frequency due to the insertion of the external inductance Ls is given by Δfr = (fr′−f
r) is represented by the following equation (4).

[0048]

(Equation 4)

Therefore, as is apparent from equation (4), S
The smaller the equivalent inductance L1 of the AW resonator or the larger the external inductance Ls, the larger the change Δfr in the resonance frequency.
On the other hand, to reduce the equivalent inductance L1, SA
It is sufficient to lower the impedance of the W resonator, and therefore, if the electrode capacitance of the resonator is increased, the equivalent inductance L
1 can be reduced. Further, as for the external inductance Ls, since the inductance by the bonding wire is necessarily introduced, the external inductance Ls can be increased by lengthening the bonding wire. As described above, since the anti-resonance frequency does not change depending on the value of the external inductance Ls, the characteristics in the pass band do not deteriorate even if the bonding wire is lengthened.

Therefore, in the surface acoustic wave filter 1 having a plurality of parallel arm resonators, the resonance frequencies of the parallel arm resonators 7 and 8 are made different from each other, so that each of the resonances in the stop band lower than the pass band. Attenuation poles due to frequency are dispersed to expand a frequency range in which the amount of attenuation is large. Moreover, as described above, the resonance frequency is increased by making the electrode capacity of the parallel arm resonator 8 larger than the electrode capacity of the parallel arm resonator 7 and making the length of the bonding wire 9D longer than the length of the bonding wire 9C. Because the difference Δfr is obtained, not only is the frequency range where the amount of attenuation on the lower side of the pass band is large is expanded, but also the respective anti-resonance frequencies do not change, thus affecting the characteristics of the pass band. The frequency range in which the amount of attenuation in the stop band is large can be extended.

FIG. 4 shows the attenuation frequency characteristic of the surface acoustic wave filter 1 by a solid line A, and the characteristic obtained by enlarging the main part thereof according to the scale on the right side is shown by a solid line B. The characteristics shown in FIG. 4 indicate that in the surface acoustic wave filter 1, the series arm resonators 3,
The number of pairs of electrode fingers of 5 = 100, electrode finger cross width = 130 μm
Where the number of pairs of electrode fingers of the series resonator 4 is 70, the cross width of the electrode fingers is 100 μm, and the number of pairs of electrode fingers of the parallel arm resonator 7 is 6
0, electrode finger cross width = 90 μm, capacitance between electrodes = 2.5 pF
The number of pairs of electrode fingers of the parallel arm resonator 8 is 60, the electrode finger intersection width is 230 μm, the capacitance between electrodes is 6.3 pF, the length of the bonding wire 9C is about 2 mm, and the length of the bonding wire 9D is about 1 mm. This shows the characteristics when

For comparison, the parallel arm resonator 8 is configured in the same manner as the parallel arm resonator 7, and the bonding wires 9C,
The solid line C in FIG. 13 shows the attenuation frequency characteristics of the surface acoustic wave filter when the length of 9D is the same, that is, about 1 mm.
Further, a characteristic obtained by enlarging the main part according to the scale on the right is shown by a solid line D. Comparing the attenuation frequency characteristic shown in FIG. 4 with the attenuation frequency characteristic shown in FIG. 13, the surface acoustic wave filter 1 increases the attenuation by about 5 dB in the stop band on the lower frequency side than the pass band. It can be seen that the frequency range of the attenuation of 23 dB can be expanded by about 40%. In other words, although the attenuation is small at the attenuation pole, the minimum value of the attenuation can be increased in the required frequency range of the stop band.

FIG. 5 is a schematic plan view for explaining a second structural example of the surface acoustic wave filter according to the present invention.
Is a diagram showing an equivalent circuit thereof. As is clear from FIG. 6, in the surface acoustic wave filter 21, two series arm resonators 22 and 23 are inserted in a series arm formed between the input terminal 2a and the output terminal 2b. Also, the input terminal 2a
A parallel arm resonator 25 is inserted in a parallel arm connected between a connection point 24a between the arm and the series arm resonator 22 and a reference potential. A parallel arm resonator 26 is connected to a parallel arm connected between a connection point 24b between the series arm resonators 22 and 23 and a reference potential. Further, a parallel arm resonator 27 is inserted in a parallel arm connected between a connection point 24c between the series arm resonator 23 and the output terminal 2b and the reference potential.

The inductances 29a to 29e indicate the inductances due to the bonding wires, respectively, and are the same as the inductances 9a to 9d shown in FIG. However, in the surface acoustic wave filter 21, the length of a bonding wire 29D described later is made longer than the bonding wires 29C and 29E so that the inductance 29d is larger than the inductances 29c and 29e.

The parallel arms connected to connection points 24a, 24c between the input / output terminals 2a, 2b and the series arm resonators 22, 23 adjacent to the input / output terminals 2a, 2b. Parallel arm resonator 2 as first parallel arm resonator
The capacitance between the electrodes 5 and 27 is made equal. On the other hand, the capacitance between the electrodes of the second parallel arm resonator 26 arranged in the parallel arm connected to the connection point 24b between the series arm resonators 22 and 23 is equal to the capacitance of the first parallel arm resonators 25 and 27. It is doubled.

As shown in FIG. 5, the surface acoustic wave filter 2
In 1, the surface acoustic wave filter element 30 configured as described above is arranged in the opening 10 a of the package material 10. Since the structure of the package material 10 is the same as that of the surface acoustic wave filter 1, the same portions are denoted by the same reference numerals and description thereof will be omitted.

In the surface acoustic wave filter 21, the series arm resonator 22 is connected to the external terminal 11b as an input terminal by a bonding wire 29A. The series arm resonator 23 is electrically connected to an external terminal 11e as an output terminal by a bonding wire 29B.
The series arm resonators 22 and 23 are each formed of a one-port SAW resonator having an IDT at the center and reflectors on both sides, similarly to the series arm resonator 3 shown in FIG. The series arm resonators 22 and 23 are connected to each other in series by a conductive pattern 31a.

Each of the parallel arm resonators 25, 26 and 27 is also constituted by a one-port SAW resonator having a reflector on both sides. The parallel arm resonator 25 has a bonding wire 29C with respect to the external terminal 11a connected to the reference potential.
Are electrically connected to each other. The parallel arm resonator 26 is
It is electrically connected to the external terminal 11d connected to the reference potential by a bonding wire 29D. Further, the parallel arm resonator 27 is electrically connected to the external terminal 11f connected to the reference potential by a bonding wire 29E.

In the surface acoustic wave filter 21, the piezoelectric substrate 1
Reference numeral 3 denotes a 36 ° Y-cut LiTaO ₃ substrate, and the above-described resonators 22, 23, 25 to 27 and the conductive pattern 31a are formed by depositing aluminum on the piezoelectric substrate 13 and patterning the same. ing.

Generally, in order to achieve impedance matching of a ladder-type filter, a combination of one series arm resonator and one parallel arm resonator is used as one block, and the input and output of a plurality of blocks are inverted to form a cascade connection. Let it. According to this method, the impedance of the parallel arm resonator disposed on the input / output end side is equal to the impedance of the parallel arm resonator of the other parallel arm.
Double. Therefore, in the equivalent circuit shown in FIG. 6, when the impedance of the second parallel arm resonator 26 is 1/2 of that of the first parallel arm resonators 25 and 27, impedance matching is achieved.

In this case, from equation (4) described above, if the bonding wire 29D connecting the parallel arm resonator 26 to the reference potential is made longer, the change in the resonance frequency of the parallel arm resonator 26 is maximized. Thus, the frequency range in which the amount of attenuation on the lower frequency side than the pass band is large can be expanded. Therefore, the surface acoustic wave filter 21
Also, in the structure in which the electrode capacitance of the parallel arm resonator 26 as the second parallel arm resonator arranged in the parallel arm connected to the connection point between the series arm resonators is relatively large, By making the length of the bonding wire connected to the reference potential of the arm resonator 26 relatively long, the attenuation amount in the stop band on the lower frequency side than the pass band is made similar to the case of the surface acoustic wave filter 1. It can be seen that a large frequency range can be extended.

FIG. 7 is a diagram showing the attenuation frequency characteristics of the surface acoustic wave filter 21, and the solid line E is a characteristic obtained by enlarging the main part of the characteristic shown by the solid line F according to the scale on the right.
This attenuation frequency characteristic indicates that the series arm resonators 22 and 23
The number of electrode finger pairs = 95, the electrode finger cross width = 60 μm, the interelectrode capacitance = 2.6 pF, the number of electrode fingers of the parallel arm resonators 25 and 27 = 80, the electrode finger cross width = 60 μm, the interelectrode capacitance = 2.2 pF, the number of pairs of electrode fingers of the parallel arm resonator 26 = 80, the electrode finger intersection width = 120 μm, and the interelectrode capacitance = 4.
42 pF and the length of the bonding wire 29D is 2 m
m, length of bonding wires 29C and 29E is 1 mm
This shows the characteristics when The characteristics shown in FIG.
As is apparent from the comparison with the characteristics shown in FIG. 6, it is understood that the attenuation can be increased by about 5 dB on the lower frequency side than the pass band in this structural example.

FIG. 8 is a schematic plan view for explaining a third structural example of the surface acoustic wave filter according to the present invention.
The surface acoustic wave filter 41 according to the third structural example has a five-element configuration of three parallel-arm resonators and two serial-arm resonators, and the physical configuration itself is the same as that shown in FIG. This is the same as the second structural example. Therefore, the corresponding parts are denoted by the same reference numerals and the description thereof will be omitted.

The surface acoustic wave filter 41 is different from the surface acoustic wave filter 21 in that the second parallel arm resonator 26
Is four times the electrode capacitance of the parallel arm resonators 25 and 27 as the first parallel arm resonator.
That is, the configuration is the same as that of the surface acoustic wave filter 21 shown in FIG. 5, except that the electrode area of the parallel arm resonator 26 is four times the electrode area of the parallel arm resonators 25 and 27.

Surface acoustic wave filter 2 according to second structural example
In No. 1, impedance matching in the pass band is achieved by making the electrode capacity of the second parallel arm resonator 26 twice the electrode capacity of the first parallel arm resonators 25 and 27. However, in the second structure example, it is necessary to add a large inductance when expanding the attenuation region. On the other hand, the size of the surface acoustic wave filter package has been reduced year by year, and there is a limit in the size of the insertion of the inductance part by the bonding wire as described above. Therefore, it is necessary to change the resonance frequency by adding a smaller inductance.

In the third structural example, in order to satisfy the above requirements, the electrode capacitance of the parallel arm resonator 26 is further increased,
By reducing the electrode capacitance of the first parallel arm resonators 25 and 27 and increasing the amount of change in the resonance frequency of the second parallel arm resonator 26, the stop band in the lower band than the pass band is reduced. It is possible to expand a frequency range in which the amount of attenuation is large. However, as described above, in order to achieve impedance matching in the pass band, the electrode capacity ratio between the second parallel arm resonator 26 and the first parallel arm resonators 25 and 27 is ideally 2: 1. If the impedance difference becomes too large, the impedance matching cannot be achieved. Therefore, the range of the practical capacity ratio is naturally limited in consideration of impedance matching.

FIG. 9 shows a change in VSWR (voltage standing wave ratio) when the electrode capacitance ratio of the central second parallel arm resonator 26 and the first parallel arm resonators 25 and 27 on both sides is changed. FIG. Generally, in a high-frequency filter, it is desirable that the VSWR is 2 or less. However, as is apparent from FIG. 9, when the electrode capacitance ratio becomes 6 times or more, the VSWR sharply increases. Therefore, it is understood that the ratio of the electrode capacity of the second parallel arm resonator to the electrode capacity of the first parallel arm resonator is desirably 6 times or less.

As described above, by increasing the electrode capacitance of the second parallel arm resonator within the range in which the value of VSWR does not exceed 2, the filter characteristics are not degraded, and the pass band is reduced to the low frequency side. The frequency range in which the amount of attenuation in the stop band is large can be expanded. Therefore, if the length of the bonding wire connected to the second parallel arm resonator is increased, the resonance frequency of the second parallel arm resonator can be further reduced. The frequency range in which the amount of attenuation is large can be expanded.

FIG. 10 shows the attenuation frequency characteristics of the surface acoustic wave filter 41. Note that the solid line G is a characteristic obtained by enlarging the main part of the characteristic indicated by the solid line H on the right scale. From FIG. 10, it can be seen that also in the surface acoustic wave filter 41 of the third structural example, the attenuation in the lower stop band can be increased by about 5 dB.

[0070]

【The invention's effect】

[0071]

The surface acoustic wave filter according to the present invention has a plurality of series arm resonators and a plurality of parallel arm resonators, and is provided between the input / output terminal and the series arm resonator adjacent to the input / output terminal. Te configuration smell parallel arms to the connection point is connected, as compared to the first parallel arm resonators inserted in the parallel arm disposed proximate to the input output connection point between the series arm resonators By increasing the electrode capacitance of the second parallel arm resonator disposed in the parallel arm connected to the power supply, the frequency range in which the amount of attenuation is large in the stop band on the low frequency side of the pass band can be expanded. Therefore, deterioration of the pass band characteristics does not occur.
Frequency in the low-frequency stopband
Since the number range can be expanded, the surface acoustic wave filter according to the present invention is
Filters are used, for example, for transmitting filters and receiving
It can be suitably used as a filter.

[0073] Furthermore, the electrode capacitance of the upper Symbol second parallel arm resonator
Is greater than twice and less than or equal to six times the electrode capacitance of the first parallel arm resonator . Therefore , while suppressing the increase in VSWR, the attenuation of the stop band on the lower frequency side than the pass band is suppressed. In order to further expand the large frequency range, it is necessary to increase the amount of attenuation in the stop band on the low frequency side of the pass band without increasing the length of the bonding wire connected to the second parallel arm resonator. This can contribute to downsizing of the surface acoustic wave filter.

[0074] the of al, the length of the bonding wire that connects the reference potential second parallel arm resonators, than bonding wires connected between the first parallel arm resonator and a reference potential Is also lengthened, so that the frequency range in which the amount of attenuation in the stop band on the lower frequency side of the pass band is large can be further expanded.

[Brief description of the drawings]

FIG. 1 is a schematic plan view for explaining one structural example of a surface acoustic wave filter according to the present invention.

FIG. 2 is a diagram showing an equivalent circuit of a structural example of a surface acoustic wave filter according to the present invention.

FIG. 3 is a diagram for explaining an equivalent circuit of a one-port SAW resonator.

FIG. 4 is a diagram showing an attenuation frequency characteristic of a surface acoustic wave filter according to one structural example of the present invention.

FIG. 5 is a schematic plan view for explaining a surface acoustic wave filter according to a second structural example of the present invention.

FIG. 6 is a diagram showing an equivalent circuit of the surface acoustic wave filter shown in FIG.

FIG. 7 is a diagram showing an attenuation frequency characteristic of the surface acoustic wave filter shown in FIG. 5;

FIG. 8 is a schematic plan view for explaining a third structural example of the surface acoustic wave filter according to the present invention.

FIG. 9 shows the electrode capacitance ratio and VS of the first and second parallel arm resonators.
The figure which shows the relationship with WR.

FIG. 10 is a diagram showing attenuation frequency characteristics of a surface acoustic wave filter according to a third structural example of the present invention.

FIG. 11 is a schematic plan view for explaining an example of a conventional surface acoustic wave filter.

FIG. 12 is an enlarged plan view for explaining a one-port SAW resonator.

FIG. 13 is a diagram showing attenuation frequency characteristics of a conventional surface acoustic wave filter.

FIG. 14 is a diagram showing an attenuation frequency characteristic of a conventional surface acoustic wave filter.

[Explanation of symbols]

 DESCRIPTION OF SYMBOLS 1 ... Surface acoustic wave filter 2a ... Input terminal 2b ... Output terminal 3, 4, 5 ... Series arm resonator 6a, 6b ... Connection point 7, 8 ... Parallel arm resonator 9a-9d ... Inductance 9A-9D ... Bonding wire 10 ... Packaging materials 11a to 11f ... External terminals 12 ... Surface acoustic wave filter element 13 ... Piezoelectric substrate 21 ... Surface acoustic wave filter 22,23 ... Series arm resonator 25-27 ... Parallel arm resonator 24a, 24b, 24c ... Connection point 29a to 29e: inductance 29A to 29E: bonding wire 41: surface acoustic wave filter

──────────────────────────────────────────────────続 き Continued on front page (58) Field surveyed (Int.Cl. ⁷ , DB name) H03H 9/145 H03H 9/64

Claims (1)

(57) [Claims] 1. A plurality of series arm resonators on a piezoelectric substrate so as to form a ladder type filter circuit having a series arm between input / output terminals and a plurality of parallel arms between the series arm and a reference potential. and Ri Na to form a plurality of parallel arm resonators, the plurality of
A parallel arm resonator is connected to the input terminal and the series close to the input terminal.
Parallel arm connected to the connection point between the arm resonator and the output
Connection between the terminal and the series arm resonator adjacent to the output terminal.
Each of the parallel arms connected to the connection point
1 connected to the connection point between the parallel arm resonator and the series arm resonator.
A second parallel arm resonator disposed on the parallel arm
A surface acoustic wave filter element, a package material accommodating the surface acoustic wave filter element, and having a plurality of external terminals connected to the surface acoustic wave filter element, and input / output of the surface acoustic wave filter element. In a surface acoustic wave filter including a terminal and a reference potential terminal, and a plurality of bonding wires respectively connecting the plurality of external terminals of the package material, the electrode capacitance of the second parallel arm resonator is the same as that of the second parallel arm resonator. 1 parallel
More than twice the electrode capacitance of the arm resonator and no more than six times
And the second parallel arm resonator, and a package
Connects to an external terminal connected to the reference potential provided on the material
The length of the bonding wire being connected is
Connected to the reference potential provided on the arm resonator and the package material
A surface acoustic wave filter which is longer than a bonding wire connecting the external terminal to be connected .